Like mammalian neurons, C. elegans neurons lose regeneration ability as they age, but it is not known why. C. elegans is a soil worm with its brain wiring diagram being mapped entirely - every connection between every nerve cell. Forty percent of genes identified in the worm genome have a counterpart in humans. Genes that allow neurons to connect with each other to form functional neural circuits and to regenerate themselves after injury are highly similar between worms and humans. Thus, what we learn in worms will likely be relevant to the development and regeneration of the human nervous system. The let-7 microRNA and the tripartite motif protein LIN-41 are well known for their roles in timing mitotic cell development required for molting in worms and embryonic stem cell self-renewal in mice, but whether they are re-used in postmitotic neurons to time their post-differentiation events is not known. Our recent results show that the let-7 microRNA and its target, the LIN-41 tripartite motif protein, function as neuronal timers in worms to time the decline of the ability of neurons to regenerate as they age. The progressive increase of let-7 and the progressive decrease of lin-41 in neurons provide intrinsic timing mechanism. Furthermore, the effect of let-7 and lin-41 in regulating neuronal regeneration is mediated through the LIN-29 transcription factor. Like C. elegans neurons, mammalian neurons also suffer from the age-related decline in regeneration ability. The idea of slowing down neuronal aging to promote regeneration is an appealing possibility. Our study has important implications in treating neurodegenerative diseases of aging as it shows that it may be possible to improve the ability of neurons in the aging brain to regenerate after diseases through therapeutic inhibition of the let-7 microRNA, and thereby restore their youthful regenerative capacity. Our preliminary results provide strong support for the three specific aims proposed in this application. Specific aim 1 is to determine how broadly the timing pathway let-7-lin-41-lin-29 regulates age-related decline in neuronal regeneration. Specific aim 2 is to identify mechanisms by which the downstream LIN-29 transcription factor regulates AVM neuronal regeneration. Specific aim 3 is to investigate mechanisms that control the timing of let-7 expression in regulating AVM neuronal regeneration. This proposal describes a comprehensive and multifaceted set of experiments aimed at understanding the mechanisms by which neuronal timing networks regulate age-related decline in neuronal regeneration. The ability to focus on mechanisms of aging in a single neuron type in vivo, and to readily employ complementary experimental approaches, will allow us to describe the required mechanisms at a resolution that is not readily attainable in other systems.